FIG. 1 is a circuit diagram showing a DC converter of this type according to a related art. In the DC converter shown in FIG. 1, a DC power source Vdc1 is connected through a primary winding 5a (the number of turns being n1) of a transformer T to a main switch Q1 which may be a MOSFET (hereinafter referred to as FET). Both ends of the primary winding 5a are connected to a parallel circuit, which is composed of a resistor R2 and a snubber capacitor C2, and a diode D3 connected in series with the parallel circuit. The main switch Q1 is turned on and off under PWM control by a control circuit 100.
The primary winding 5a and a secondary winding 5b of the transformer T are wound so as to generate in-phase voltages. The secondary winding 5b (the number of turns being n2) of the transformer T is connected to a rectifying/smoothing circuit composed of diodes D1 and D2, a reactor L1, and a capacitor C4. The rectifying/smoothing circuit rectifies and smoothes a voltage (on/off-controlled pulse voltage) induced on the secondary winding 5b of the transformer T and provides a DC output for a load RL.
The control circuit 100 has an operational amplifier (not shown) and a photocoupler (not shown). The operational amplifier compares an output voltage of the load RL with a reference voltage. If the output voltage of the load RL is equal to or greater than the reference voltage, the ON-width of a pulse applied to the main switch Q1 is controlled to be narrower. Namely, when the output voltage of the load RL becomes equal to or greater than the reference voltage, the ON-width of a pulse applied to the main switch Q1 is narrowed to maintain a constant output voltage.
Operation of the DC converter with the above-mentioned configuration will be explained with reference to a timing chart shown in FIG. 2. In FIG. 2, the main switch Q1 has a terminal voltage Q1v, passes a current Q1i, and is on/off-controlled according to a Q1-control signal.
At time t31, the Q1-control signal turns on the main switch Q1, and the DC power source Vdc1 passes the current Q1i through the primary winding 5a of the transformer T to the main switch Q1. This current linearly increases as time passes up to time t32. The primary winding 5a passes a current n1i that linearly increases as time passes up to time t32, like the current Q1i. 
Between time t31 and time t32, the primary winding 5a is negative on the main switch Q1 side, and the primary winding 5a and secondary winding 5b are in-phase. Accordingly, an anode of the diode D1 is positive to pass a current in order of 5b, D1, L1, C4, and 5b. 
Next, at time t32, the main switch Q1 is changed from the ON state to an OFF state according to the Q1-control signal. At this time, among induced energy on the primary winding 5a of the transformer T, the induced energy of a leakage inductance Lg (inductance not coupled with the secondary winding 5b) is not transferred to the secondary winding 5b and is accumulated in the snubber capacitor C2 through the diode D3.
Between time t32 and time t33, the main switch Q1 is OFF, and therefore, the current Q1i and the current n1i passing through the primary winding 5a become zero. Between time t32 and time t33, a current is passed in order of L1, C4, D2, and L1 to supply power to the load RL.
This DC converter inserts the snubber circuit (C2, R2) to relax a temporal change in the voltage of the main switch Q1, thereby reducing switching noise and suppressing a surge voltage from the leakage inductance Lg of the transformer T to the main switch Q1.